Gas powered auto-injectors and methods for use

A method for delivering one or more agents into a subject's body, comprising:

• providing an injection device comprising an outer housing comprising a proximal end and a distal end;
• an activation cap mounted on the distal end of the housing such that a contact surface is disposed distal to the distal end of the housing;
• a drive assembly slidable within the housing comprising a proximal portion adjacent the proximal end of the housing including a first chamber therein, a distal portion including a second chamber, and defining a proximal chamber between the proximal portion and the housing communicating with the first chamber;
• a syringe on a distal end of the drive assembly such that a needle of the syringe is disposed adjacent the activation cap within the distal end of the housing;
• a gas canister within the first chamber;
• a plunger comprising a proximal end within the second chamber of the drive assembly and a distal end coupled to a piston of the syringe;
• and an opener pin adjacent an outlet of the gas canister;
• placing the contact surface against the subject's skin;
• and
• pressing the device to cause the activation cap to move proximally to direct the drive assembly proximally within the housing to cause the opener pin to open the outlet of the gas canister to release pressurized gas into the first chamber and the proximal chamber, whereupon
• the pressurized gas enters the proximal chamber of the drive assembly sealed by a pair of proximal seals configured such that the pressurized gas generates a distal force to advance the drive assembly distally to direct the needle out the distal end of the housing into the subject's skin;
• the pressurized gas enters the second chamber from the first chamber proximal to the proximal end of the plunger to direct the plunger distally from an initial position towards a final position to deliver the one or more agents from the syringe through the needle into the subject;
• and
• when the plunger reaches the final position, the pressurized gas enters a distal chamber sealed by a pair of distal seals such that the pressurized gas generates a proximal force to retract the drive assembly proximally to direct the needle back into the distal end of the housing.